Figure 23–18 The Internal Structure of a Leaf Cuticle Veins Epidermis Palisade mesophyll Xylem Vein Phloem Spongy mesophyll Epidermis Stoma Guard cells

Leaf Structures & Functions Epidermis – layer of tough, irregularly shaped cells; covered by cuticle; form waterproof barrier that protects tissues and limits water loss through evaporation Mesophyll – photosynthesis in most plants occurs here Palisade mesophyll – tall columnar, cells; closely packed Spongy mesophyll – loose; many air spaces that connect to exterior through stomata

Stomata – porelike openings in the underside of the leaf; allows carbon dioxide and oxygen to diffuse into and out of leaf Guard cells – control the opening and closing of stomata by responding to changes in water pressure Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water In general, stomata are open during the daytime when photosynthesis is active, and closed at night. May be closed during hot, dry conditions to reduce water loss.

Vein – vascular tissue connected to vascular tissue of stems; transport system xylem – carries water from roots to rest of plant phloem – carries nutrients and food made by photosynthesis

Function of Guard Cells Section 23-4 Guard cells Guard cells Inner cell wall Inner cell wall Stoma Stoma Open Stoma Closed

Function of Guard Cells Section 23-4 Guard cells Guard cells Inner cell wall Inner cell wall Stoma Stoma Open Stoma Closed